Wind turbines for generating power have become increasingly commonplace in recent years. However, wind turbines are often located offshore in a position which maximises the amount of power which can be generated from the turbine. Using an offshore location for the wind turbine also reduces the visual impact of the turbine since the offshore location is generally located away from population centres.
Although an offshore location for a wind turbine may be desirable for the above mentioned reasons; this presents a difficulties in terms of building and maintaining a wind turbine. This may be particularly the case for larger wind turbines which may be 100 metres or so in height and hundreds of kilometres offshore.
Current barges for use with wind turbines include a floating platform or jack-up vessel having one or more cranes. In operation, the crane may be used in construction of the wind turbine to lift various parts of the wind turbine into position. However, particularly for larger wind turbines, moving wind turbine components from the barge to their location on the wind turbines has a number of problems. Firstly, this operation is rather weather dependent since components need to be lifted from the deck of the mobile barge to their position on the wind turbine, which may be in excess of 100 meters above the sea level. For example, in windy conditions or in rough sea conditions it may be impossible to use a crane to lift the various components into position. Furthermore, even in good conditions, the use of a crane to lift components into position is difficult and dangerous due to communication difficulties between the crane operator and crew on the barge and on the turbine. In the event of failure of the crane, serious accidents can occur which may compromise the safety of the crew, work teams, and even the turbine tower and vessel itself.
Published UK patent application number GB 2350326 A describes a method of recovering material from the seabed using a jack-up rig. The rig has upper and lower decks which are independently moveable. In order to recover material from the seabed, buoyancy tanks within the lower deck are flooded by pumping water from the sea to lower the lower deck towards the seabed. Umbilical lines L connect ancillary equipment on the lower deck with the control systems of the upper deck, thereby allowing controlled movement of the lower deck.
Published Japanese patent application number JP58000510 A describes how the stability of an offshore working platform having a number of legs may be improved. The platform has an upper working base and a lower working base which are vertically moveable relative to the legs. After the legs are landed on the seabed, a lower working base is lowered to improve the stability of the platform.
However, the rig and platform described above suffer from a number of problems which make them unsuitable for installing or repairing an offshore structure. Firstly, they are not self propelled and must be manoeuvred into position with a tug. This, however, does not allow the position of the vessel to be precisely controlled, which is necessary when installing or repairing an offshore structure.
Secondly, although each of the lower and upper decks includes means to lower or raise itself with respect to the legs so that the upper and lower decks are moveable independently of each other, this mode of operation is not in general suitable for raising heavy components above the water's surface so that they can be installed or replaced on tall offshore structures such as wind turbines. Further, it is not possible to raise components above the sea level by pumping air into buoyancy tanks flooded with water. This method of recovering material from the seabed is also problematic regarding lateral stability of the load unless the load is very evenly distributed or unless stability tanks are provided. As a result, the lowerable deck can often jam on the legs.